feat(python): add facilitator HTTP server

[Bortlesboat]

Summary

The Python SDK has x402Facilitator (logic) and HTTPFacilitatorClient (client), but no way to host a facilitator as an HTTP service. The Go SDK has this via go/http/server.go. This PR fills that gap.

Adds create_facilitator_app() — a factory function that wraps any x402Facilitator or x402FacilitatorSync instance in a FastAPI application exposing the standard facilitator endpoints:

• GET /supported — capability discovery
• POST /verify — payment verification
• POST /settle — payment settlement
• GET /health — liveness check

Design decisions

• Backend-agnostic — wraps any registered mechanism (EVM, SVM, or future mechanisms), not tied to a specific chain
• Async + sync — accepts both x402Facilitator (async) and x402FacilitatorSync (sync via thread pool)
• V1/V2 routing — handles both protocol versions based on x402Version in the request body
• Optional CORS — cors_origins parameter for cross-origin deployments
• Reuses existing types — VerifyResponse, SettleResponse, SupportedResponse from x402.schemas
• Lazy FastAPI import — only imported when create_facilitator_app() is called, so the module doesn't force the dependency

Usage

from x402 import x402Facilitator
from x402.http.facilitator_server import create_facilitator_app

facilitator = x402Facilitator()
facilitator.register(["eip155:8453"], some_scheme)

app = create_facilitator_app(facilitator)

# uvicorn app:app --host 0.0.0.0 --port 8402

Testing

20 unit tests covering:

• All four endpoints (supported, verify, settle, health)
• Valid and invalid payments
• Missing/malformed request bodies
• Unsupported scheme/network errors
• Both async and sync facilitator variants
• CORS configuration and absence
• Response schema round-trip validation (server output parseable by VerifyResponse/SettleResponse/SupportedResponse)

20 passed in 0.54s

[cb-heimdall]

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